1. Field of the Invention
This invention relates to demetalation and desulfurization of petroleum oils, particularly residual hydrocarbon components, and having a significant metals and sulfur content. More particularly the invention relates to a demetalation-desulfurization process for reducing high metals and sulfur contents of residual hydrocarbon fractions with high yield of gasoline and distillate.
2. Description of the Prior Art
Because of the large amounts of sulfur-bearing oils which are currently being employed as raw materials in the petroleum refining industry, the problems of air pollution, particularly with regard to sulfur oxide emissions, have become of increasing concern. This has become particularly true with regard to several high sulfur content feedstocks, particularly those including high-boiling asphaltene components. For these reasons various methods for the removal of sulfur from these feedstocks have been the subject of intensive research efforts by this industry. At present, the most practical commercial means of desulfurizing such fuel oils is the catalytic hydrogenation of sulfur-containing molecules and petroleum hydrocarbon feeds in order to effect the removal, as hydrogen sulfide, of the sulfur-containing molecules therein. These processes generally require relatively high hydrogen pressures, generally ranging from about 2000 to 3000 psig, and elevated temperatures generally ranging upward from 650.degree. F., depending upon the feedstock employed and the degree of desulfurization required.
Such catalytic processes are generally quite efficient for the desulfurization of distillate-type feedstocks, but become of increasing complexity and expense, and decreasing efficiency, as increasingly heavier feedstocks, such as whole or topped crudes and residua are employed. This is particularly true with regard to asphaltene-containing feedstocks, including residuum feedstocks, since such feedstocks are often contaminated with heavy metals, such as nickel, vanadium and iron, as well as with the asphaltenes themselves, which tend to deposit on the catalyst and deactivate same. Furthermore, a large portion of the sulfur content in these feeds is generally contained in the higher molecular weight molecules, which can only be broken down under the more severe operating conditions, and which generally diffuse with difficulty through the catalyst pores.
Residual petroleum oil fractions such as those heavy fractions produced by atmospheric and vacuum crude distillation columns, are typically characterized as being undesirable as feedstocks for most refining processes due primarily to their high metals and sulfur content. The presence of high concentrations of metals and sulfur and their compounds precludes the effective use of such residua as chargestocks for cracking, hydrocracking and coking operations as well as limiting the extent to which such residua may be used as fuel oil. Perhaps the single most undesirable characteristic of such feedstocks is the high metals content. Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper also sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. As the great majority of these metals when present in crude oil are associated with very large hydrocarbon molecules, the heavier fractions produced by crude distillation contain substantially all the metal present in the crude, such metals being particularly concentrated in the asphaltene residual fraction. The metal contaminants are typically large organo-metallic complexes such as metal prophyrins and asphaltenes.
At present, cracking operations are generally performed on petroleum fractions lighter than residua fractions. Such cracking is commonly carried out in a reactor operated at a temperature of about 800.degree. to 1000.degree. F., a pressure of about 1 to 5 atmospheres, and a space velocity of about 1 to 10 WHSV. Typical cracking chargestocks are coker and/or crude unit gas oils, vacuum tower overhead etc., the feedstock having an API gravity range of between about 15 to about 45. As these cracking chargestocks are lighter than residual hydrocarbon fractions, (residual fractions being characterized as having an API gravity of less than about 20) they do not contain significant proportions of the heavy and large molecules in which the metals are concentrated.
When metals are present in a cracking unit chargestock such metals are deposited on the cracking catalyst. The metals act as a catalyst poison and greatly decrease the efficiency of the cracking process by altering the catalyst so that it promotes increased hydrogen production.
The amount of metals present in a given hydrocarbon stream is generally judged by petroleum engineers by making reference to a chargestock's "metals factor." This factor is equal to the summation of the metals concentration in parts per million of iron and vanadium plus ten times the amount of nickel and copper in parts per million. The factor may be expressed in an equation form as follows: EQU F.sub.m =Fe+V+10(Ni+Cu)
A chargestock having a metals factor greater than 2.5 is indicative of a chargestock which will poison cracking catalyst to a significant degree. A typical Kuwait crude generally considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual faction of a crude stock, it is clear that metals removal of 90 percent and greater will be required to make such fractions (having a metals factor of about 150 to 200) suitable for cracking chargestocks.
Sulfur is also undesirable in a process unit chargestock. The sulfur contributes to corrosion of the unit mechanical equipment and creates difficulties in treating products and flue gases. At typical cracking conversion rates, about one half of the sulfur charged to the unit is converted to H.sub.2 S gas which must be removed from the light gas product, usually by scrubbing with an amine stream. A large portion of the remaining sulfur is deposited on the cracking catalyst itself. When the catalyst is regenerated, at least a portion of this sulfur is oxidized to form SO.sub.2 and/or SO.sub.3 gas which must be removed from the flue gas which is normally discharged into the atmosphere.
Such metals and sulfur contaminants present similar problems with regard to hydrocracking operations which are typically carried out on chargestocks even lighter than those charged to a cracking unit, and thus typically having an even smaller amount of metals present. Hydrocracking catalyst is so sensitive to metals poisoning that a preliminary or first stage is often utilized for trace metals removal. Typical hydrocracking reactor conditions consist of a temperature of 400.degree. to 700.degree. F. and a pressure of 1000 to 3500 psig.
In the past, and to a limited extent under present operating schemes, high molecular weight stocks containing sulfur and metal have often been processed in a coker to effectively remove metals and also some of the sulfur, the contaminants remaining in the solid coke. Coking is typically carried out in a reactor or drum operated at about 800.degree. to 1100.degree. F. temperature and a pressure of 1 to 10 atmospheres wherein heavy oils are converted to lighter gas oils, gasoline, gas and solid coke. However, there are limits to the amount of metals and sulfur that can be tolerated in the product coke if it is to be saleable. Hence, there is considerable need to develop economical as well as efficient means for effecting the removal and recovery of metallic and non-metallic contaminants from various fractions of petroleum oils so that conversion of such contaminated charges to more desirable product may be effectively accomplished.
It has been proposed to improve the salability of high sulfur content, residual-containing petroleum oils by a variety of hydrodesulfurization processes. However, difficulty has been experienced in achieving an economically feasible catalytic hydrodesulfurization process, because notwithstanding the fact that the desulfurized products may have a wider marketability, the manufacturer may be able to charge little or no premium for the low sulfur desulfurized products, and since hydrodesulfurization operating costs have tended to be relatively high in view of the previously experienced, relatively short life for catalysts used in hydrodesulfurization of residual-containing stocks. Short catalyst life is manifested by inability of a catalyst to maintain a relatively high capability for desulfurizing chargestock with increasing quantities of coke and/or metallic contaminants which act as catalyst poisons. Satisfactory catalyst life can be obtained relatively easily with distillate oils, but is especially difficult to obtain when desulfurizing petroleum oils containing residual components, since the asphaltene or asphaltic components of an oil, which tend to form disproportionate amounts of coke, are concentrated in the residual fractions of a petroleum oil, and since a relatively high proportion of the metallic contaminants that normally tend to poison catalysts are commonly found in the asphaltene components of the oil.
The greatest deterrent to hydrotreating of residual stocks has been the high costs for construction and operation at the pressures required for reasonable on-stream periods, usually 2000 to 3000 psig.